In precision machines, such as machine tools, coordinate measuring machines, and other like machines, gas bearings are frequently used to permit relatively frictionless movement of portions of the machine, including a carriage, along machine rails. Such gas bearings may be caused to vibrate by vibrations in the machine structure and these gas bearings, particularly ones with high static stiffness respond to machine vibrations much like a spring Such vibrations are undesirable , and can affect machine accuracy if not quickly damped out of the system. Of most concern, is when the frequency range over which the gas bearings are unstable coincides with the natural or resonant frequency of the machine parts. In such circumstances the bearing may act as an amplifier, sustaining or even amplifying the machine vibrations. Thus, it becomes even more difficult to damp the vibrations out of the system, thus destroying its alignment and accuracy.
Such vibrations are of particular concern in high precision positioning machines, such as high precision tooling machines or coordinate measuring machines. Gantry-type support structures are commonly used in such coordinate measuring machines. Such a gantry-type structure typically includes a base, a table resting on the base, a gantry structure which rides on parallel, spaced rails supported by the base and a carriage which rides on the gantry structure. A vertically movable element, typically referred to as a Z-rail, includes a touch sensitive probe disposed on the lower end thereof, and the Z-rail moves vertically with respect to the carriage, while the carriage moves horizontally along a rail disposed on the gantry structure. Gas bearings are used to facilitate the movement of the gantry structure on its associated rails. The probe disposed on the end of the Z-rail therefore can be moved in three dimensions to be positioned at any point on the table for measuring a position on a part to be tested.
In use, the gantry-type structure is rapidly accelerated and decelerated to deliver the probe to the point at which it is to perform its work or measurement. Such rapid movements are necessary so that measurements can be taken rapidly and so that the period for testing a part is not intolerably long. In part because such gantry-type structures generally are driven only at one end, vibrations can result in the structure because of the structure's inherent inertia, as described below. A torque is produced about the point at which the drive is coupled to the gantry-type structure, and it may be assumed for most purposes that the torque is applied at the center of gravity of the gantry-type structure and has a moment arm which extends from the point of coupling of the drive to the structure to the center of gravity thereof. Since the gantry-type structure has a certain elasticity, and since continued movement of the structure is resisted by the drive, and by the bearings which maintain the proper positioning of the structure, this torque causes vibrations to be set up in the gantry about the point at which the gantry is coupled to the drive. These vibrations are then transferred to the air bearings, which as described, act like an active string. Obviously, it is desirable to damp the vibrations out of the system as quickly as possible to avoid measurement errors.
For high precision coordinate measuring machines, it is essential that the measurements be taken when no vibrations are present, so that the precise alignment can be maintained. Therefore most prior art coordinate measuring machines have bearings which provide high static stiffness and a somewhat lower dynamic stiffness. This high static stiffness is achieved by making the air gap suite narrow and the gas supply low. As the bearing moves from a static condition to a dynamic one in which it is moving with respect to its associated rail, the stiffness of the bearing decreases with respect to the vibrational frequency of the structure until a generally constant, steady state stiffness is reached. Should the natural or resonant frequency of the machine structure coincide with this area of declining stiffness, the system could become unstable, and the vibrations would be very difficult to damp out of the system.
In an effort to reduce the frequencies over which the stiffness versus frequency curve has a negative slope, some prior art bearing shoes are provided with a small internal reservoir communicating with the gas ports which lead to the bearing surface of the shoe and which provide the desired gas cushion. This internal reservoir damps certain high frequency vibrations, and at these frequencies, this chamber acts like an unlimited air supply, so that the bearing appears less stiff to damp the vibrations more quickly. However, this prior art internal reservoir is not sufficient to damp vibrations at certain lower frequencies. In certain gantry-type coordinate measuring machines, because of their large moments of inertia, the system has resonant frequencies which are lower than those that can be damped by such prior art bearings. These resonant frequencies coincide with the negative part of the slope of the stiffness versus frequency curve and the system can become unstable under those conditions.
Therefore, it is an object of the present invention to provide a gas bearing with improved stiffness.
It is another obJect of the present invention to provide an improved gas bearing for use in high precision coordinate measuring machines having a gantry-type structure.
It is yet another object of the present invention to provide a gas bearing which provides adequate damping over a broader range of frequencies.
It is yet another further obJect of the present invention to provide a gas bearing with high static stiffness which dampens vibrations at all frequencies which coincide with the resonant frequencies of a gantry-type coordinate measuring machine.
It is still another further object or the present invention to shift the portion of the bearing stiffness versus frequency curve having a negative slope into an area where there are no resonant frequencies for a gantry-type coordinate measuring machine.